Perfect, spotless, all good.
Confirms that something is sorted, and also describes something genuinely immaculate. "Nickel!" on its own means "perfect."
From the metal — nickel-plated surfaces were the byword for spotlessly clean.
Reading a definition isn't the same as saying it to someone. Lingo Kaiava is a voice tutor set to French — it uses nickel the way locals do, corrects you in the moment, and never "fixes" the way real people actually talk into textbook French.
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